Year 4 pupils put the teacher on the spot by asking him 'awkward' questions about the existence of Father Christmas, but instead of telling the children what we parents tell ours (the truth, obviously), the man said the large, round, bearded chap that flies through the air pulled by reindeer to bring them presents wasn't real.
The teacher was taking a literacy lesson at a primary school in Christchurch, Dorset, which involved the kids writing a letter to Father Christmas.
Headteacher Simon Adorian told MailOnline the teacher had made a mistake.
"He responded clumsily and didn't do his best," he said.
"He was asked whether he existed and responded in a way, and with a look, that gave them doubt.
"He made a mistake, but we have covered it up, just as you do with children and they haven't asked about it anymore.
"I have probably never spoken to someone who is feeling as sorry as this teacher.
"He is extremely sorry about it, nothing like this has ever happened before, we celebrate Christmas as traditionally as other schools."
The head added that the teacher was 'mortified'.
But the explanation hasn't quelled the anger of parents.
One said: "I was fuming when I found out. I spoke to the teacher and to the headteacher.
"The headteacher apologised, but the teacher seemed shocked that people were complaining.
"He told the children to put their hands over their ears if they did not want Christmas spoilt, so it was not accidental."
One parent, who asked not to be named, said: "This teacher has upset all our children. Children left the room saying to their parents that Santa Claus isn't really real, which upset them, and that is the last thing I want."